This invention relates to electrically heating a body of molten material by means of induction. The invention is applicable to the melting of a material which is susceptible to having electric currents induced therein, and it is particularly applicable to the melting of glass or the like.
It is well known that material may be heated by induced currents when the material is placed within a coil carrying alternating current. An advantage of this type of heating is that the material being heated does not come into contact with the electrical source; e.g., electrodes need not be immersed in the melt. The general concept of inductively heating glass has been disclosed in many patents, for example, U.S. Pat. Nos. 1,830,481; 1,906,594; 3,205,292; and 3,244,495. A major difficulty with induction melting, however, is the provision of a suitable container for the melt. Accordingly, much of the prior art is limited to small scale embodiments, and large-scale melting of glass by induction heating has not found significant commercial acceptance. Since the vessel lies within the electric field of the induction coil, the vessel itself may suscept induced currents and become heated. This is generally undesirable because electrical power is dissipated in the vessel rather than in the material which is intended to be heated, and because heating the vessel can thermally damage the vessel and accelerate erosion of the vessel which may contaminate the product material. A non-metallic vessel can be cooled externally to maintain its temperature below that at which it is significantly susceptible to induced currents, but the cooling can extract significant amounts of thermal energy from the melting process. Also, cooling the exterior of a ceramic vessel can produce temperature gradients that can lead to large thermal stresses sufficient to damage the vessel. With a multi-pieced ceramic vessel fabricated from a plurality of pieces, however, containment of the molten material becomes a problem, particularly with a large scale operation. Use of metallic bracing such as steel or the like to maintain the structural integrity of a multi-piece ceramic vessel is undesirable because the stray currents induced in the bracing substract power from the melting operation. A metallic vessel on the other hand, is highly susceptible to induced currents, and therefore, major power losses will result even if it is cooled. Moreover, cooling a metallic vessel results in high heat losses due to the high thermal conductivity of metal. An example of a cooled metal induction heating vessel is shown in U.S. Pat. No. 3,461,215 of Reboux.